Array antennas have numerous antenna or radiating elements spaced apart from one another in a predetermined configuration. The radiation received at or transmitted from the numerous elements combines so that the antenna array acts as a combined unit with directional characteristics. In particular, when an RF signal exhibits diverse phases as it passes through the numerous antenna elements of an array, a collective RF signal forms a beam that may be directed at a particular azimuth and/or elevation angle.
It is well known that by independently controlling phase shifts applied to signals passed through numerous antenna elements, antenna beams may be electronically formed or steered in desired directions. Antennas arrays which use such phase shifting are referred to as phased array antennas. However, when the product of the signals' fractional bandwidth and the tangent of the steering angle divided by the antenna beamwidth exceeds approximately 0.1, an undesirable problem known as squint begins to pose a serious problem.
Briefly, a given phase shifter shifts the phases of different frequency signals by the same amount when it is desired that the phase shift be proportional to the frequency thereby generating a constant time delay for all frequencies. Thus, a beam formed by a phased array antenna for a wide bandwidth signal is smeared over a range of azimuth and/or elevation angles, with frequency components farthest from the center frequency exhibiting the greatest deviation from the desired beam steering angle. Squint worsens as bandwidth increases, physical array size increases, and steering angle increases.
The problem of squint may be solved by replacing phase shifters in an antenna array with delay generators. While this solution is known in theory, practical implementations of time-delayed antenna arrays for electromagnetic signals have been difficult to achieve. Electromagnetic signals propagate at extreme speed, i.e. the speed of light. Electronic components which are compatible with this extreme speed and the needs of an antenna array are not conventionally available. In particular, the problem relates to the physical length of the propagation path required to obtain delay and the need to vary this length in small enough increments that beam quality is not degraded. Maximum propagation path lengths required are on the order of the antenna dimensions. Large arrays require large and extremely precise delays. Accordingly, conventional implementations of time-delayed array antennas have been limited to sub-array configurations of switched transmission line elements which compromise antenna performance and become increasingly infeasible as array size increases.